A semiconductor device has a deep trench structure with a dielectric liner on sidewalls of the deep trench and doped polycrystalline silicon (polysilicon) on the dielectric liner filling the deep trench. Attaining a desirably low sheet resistance in the polysilicon in the deep trench requires in situ doping as the polysilicon is deposited, which undesirably causes dopant contamination on the backside of the substrate of the semiconductor device and stress in the semiconductor device after the deposited polysilicon is annealed. Both undesirable effects can degrade performance and reliability of the semiconductor device. Alternatively, undoped polysilicon may be deposited and implanted at the top surface of the semiconductor device, requiring a long thermal drive to attain a desired uniformity of dopant distribution in the deep trench, which may be over 20 microns deep. The long thermal drive adversely affects doped structures in the substrate, such as buried layers.